1. Field of the Invention
This invention relates to a device to control the intensity or direction of light from independent light sources, and in particular, such a device useful in a backlight source of liquid crystal display (LCD) devices.
2. Description of the Related Art
A typical LCD device includes a light source system with a number of monochromatic light sources, such as and not limited to, red light source, green light source, and blue light source, generating red (R), green (G), and blue (B) lights, respectively. Because the liquid crystal can only modulate the light intensity but cannot produce light itself, the light source system is generally a surface light source that is placed behind and illuminates the liquid crystal panel, commonly referred to as backlight source.
Currently, the following schemes of LCD devices and their backlight sources are in use.
The first one uses a light source side-coupling method. Its structural mechanism is shown in FIG. 1. The light source is placed at the side of a wedge-shaped light guide plate. The upper surface of this light guide plate generally has some microstructures for light scattering. This way, when the light is injected into the light guide plate from the side, it transmits out from the upper surface while propagating forward, and enters transmission type or semi-transmission/semi-reflection type liquid crystal panel 4. The microstructures help to scatter the outgoing light from the surface light source. The light source can be cold-cathode fluorescent lamps (CCFL) often used in traditional CCFL displays, or solid state light source, such as and not limited to, light emitting diodes (LED). For example, for most of today's mobile phones, the backlights of their liquid crystal screen use white LEDs.
The second scheme is the light source back-coupling method. Its structural mechanism is shown in FIG. 2. A light source, for example an array of LEDs 1, is placed at the back of the liquid crystal panel 4 to illuminate the liquid crystal panel directly. To solve the problem of uniformity of the outgoing light from the surface light source, a scattering body 51 and a scattering surface 52 are often inserted in between the array of LEDs 1 and the liquid crystal panel 4 to form a sufficiently large scattering space to achieve a uniform distribution of the outgoing light. U.S. Pat. No. 7,052,152 B2 discloses such a backlight source which also includes a layer of wavelength conversion material. The wavelength conversion material converts excitation light from the array of LEDs into excited light and provides the excited light to illuminate the liquid crystal panel. This patent also discloses various electrical connections of respective LED in the array of LEDs.
The third scheme is a combination of the light source side-coupling and the light source back-coupling method. For example, Chinese patent application No. 03123095 discloses a backlight module including a light guide plate, a side light source, and a back light source.
There are shortcomings for the existing techniques mentioned above. For the first scheme of using light source side-coupling method, it is difficult for the light guide plate to ensure that the liquid crystal panel is uniformly illuminated when the size of liquid crystal panel is large. The display brightness of the portion of the screen that is far away from the side light source is especially difficult to be controlled or guaranteed. Even adopting double side-coupling for improvement, the size of the liquid crystal panel is still limited due to the non-uniformity of display brightness. Besides, the total thickness of this type of backlight source depends on the width of the light source and the thickness of the light guide plate, and thus it is difficult to make it ultra thin.
For the second scheme of using light source back-coupling method, although it can overcome the screen size-limit problem mentioned above, the necessary existence of the scattering body or the scattering space not only reduces the light transmission efficiency but also increases the total thickness of the backlight module. If the scattering body or the scattering space is reduced in order to make the device ultra thin, the occurrence of red, blue or green light spots may be unavoidable due to the insufficient mixture of red, green, and blue light.
For the third scheme of combining the side-coupling and the back-coupling method, although it overcomes the shortcoming of the previous two methods, there is still some degree of thickness requirement for the backlight source.